научная статья по теме INITIATION OF TRANSFORM FAULTS AT RIFTED CONTINENTAL MARGINS: 3D PETROLOGICAL-THERMOMECHANICAL MODELING AND COMPARISON TO THE WOODLARK BASIN Геология

Текст научной статьи на тему «INITIATION OF TRANSFORM FAULTS AT RIFTED CONTINENTAL MARGINS: 3D PETROLOGICAL-THERMOMECHANICAL MODELING AND COMPARISON TO THE WOODLARK BASIN»

nETPomma, 2013, moM 21, № 6, c. 600-6II

INITIATION OF TRANSFORM FAULTS AT RIFTED CONTINENTAL MARGINS: 3D PETROLOGICAL-THERMOMECHANICAL MODELING AND COMPARISON TO THE WOODLARK BASIN

© 2013 T. V. Gerya

Institute of Geophysics, ETHZürich, Sonneggstrasse 5, 8092Zürich, Switzerland; e-mail: taras.gerya@erdw.ethz.ch

Received January 22, 2013; in final form, April 3, 2013

Abstract — This work presents high-resolution 3D numerical model of transform fault initiation at rifted continental margins. Our petrological-thermomechanical visco-plastic model allows for spontaneous nucleation of oceanic spreading process in a continental rift zone and takes into account new oceanic crust growth driven by decompression melting of the asthenospheric mantle. Numerical model predicts that ridge-transform spreading pattern initiate in several subsequent stages: crustal rifting (0—1.5 Myr), spreading centers nucleation and propagation (1.5—3 Myr), proto-transform fault initiation and rotation (3—5 Myr) and mature ridge-transform spreading (>5 Myr). Comparison of modeling results with the natural data from the Wood-lark Basin suggests that the development of this region closely matches numerical predictions. Similarly to the model, the Moresby (proto-) Transform terminates in the oceanic rather than in the continental crust. This fault associates with a notable topographic depression and formed within 0.5—2 Myr while linking two offset overlapping spreading segments. Model reproduces well characteristic "rounded" contours of the spreading centers as well as the presence of a remnant of the broken continental crustal bridge observed in the Woodlark Basin. Proto-transform fault traces and truncated tip of one spreading center present in the model are also documented in nature. Numerical results are in good agreement with the concept of Taylor et al. (2009) which suggests that spreading segments nucleate en echelon in overlapping rift basins and that transform faults develop as or after spreading nucleates. Our experiments also allow to refine this concept in that (proto)-transform faults may also initiate as oblique rather than only spreading-parallel tectonic features. Subsequent rotation of these faults toward the extension-parallel direction is governed by space accommodation during continued oceanic crust growth within offset ridge-transform intersections.

DOI: 10.7868/S0869590313060034

INTRODUCTION

Origin and dynamics of oceanic transform faults initiation remains a matter of debates (e.g., Gerya, 2012 and references therein). One common view is that oceanic transform faults are inherited from the continental plate breakup (e.g., Wilson, 1965; Lister et al., 1986; Cochran and Martinez, 1988; McClay and Khalil, 1998; Behn and Lin, 2000). However, an alternative view exists (Bosworth, 1986; Rosendahl, 1987; Taylor et al., 1995, 2009) that the characteristic orthogonal ridge — transform fault pattern is not directly inherited from the earlier rift geometry. This view is, in particular, supported by recent high-resolution bathymetry data from the incipient oceanic spreading regions such as Woodlark Basin, Gulf ofAden and NW Australia showing that initial offsets between oceanic spreading centers, where present, are typically non-transform (Taylor et al., 1995, 2009).

Due to the limited availability of natural data, detailed interpretations of nucleation and evolution of ridge-transform oceanic spreading patterns are difficult and controversial (e.g., Lister et al., 1986; Bosworth, 1986; Taylor et al., 1995, 2009). In addition to natural studies, a number of analogue and numerical

models of rifting and spreading have been developed and investigated (see recent review by Gerya, 2012 and references therein). Analogue models comprised (i) themomechanical freezing wax models with accreting and cooling plates (Oldenburg and Brune, 1972; O'Bryan, et al., 1975; Ragnarsson et al., 1996; Katz et al., 2005) and (ii) purelly mechanical models with brittle lithosphere and viscous mantle (Dauteuil and Bran, 1993; Dauteuil et al., 2002; Marques et al., 2007; Tentler and Acocella, 2010). Both types of models are proven to be useful for analyzing physics and dynamics of spreading process but have certain dissimilarities to nature (Gerya, 2012, 2013 and references therein): the freezing wax models often produced open spreading centers with exposed liquid wax, whereas new lithosphere is not accreted in purely mechanical models. Due to the growing computational power numerical models of oceanic spreading have recently become increasingly popular (Gerya, 2012 and references therein). Initially, numerical models of oceanic spreading (Hieronimus, 2004; Choi et al., 2008) and continental rifting (Allken et al., 2011, 2012) patterns focused on relatively short-term plate fragmentation (rifting) processes and demonstrated

Fig. 1. Initial model setup and boundary conditions for 3D petrological-thermomechanical numerical model. Boundary conditions are constant spreading rate in x-direction (vspreading = vjeft + vright, where vjeft = vright) and compensating vertical influx velocities through the upper and lower boundaries (vtop and vbottom) are chosen to ensure conservation of volume of the model domain and constant average 5 km thickness of the sea water layer [(vtop+ vbottom)/50 = (v[eft + vright)/202), where vtop/5 = vbottom/45]; front and back boundaries in the x-y plane are free slip. A water-loaded free surface condition for the upper plate boundary is implemented by using a weak layer approach (e.g., Schmeling et al., 2008; Gerya, 2010b; Crameri et al., 2012): the weak 5 km thick sea water layer has a characteristic density of 1000 kg/m3 and a viscosity of 1018 Pa s to ensure small stresses (<105 Pa) along the upper plate interface. The symmetric initial thermal structure is perturbed in two places where offset linear thermal anomalies (weak seeds) A and B are prescribed by gently elevated geotherm. Thermal boundary conditions are insulating (zero heat flux) on all boundaries with except of the upper and lower boundaries, over which a constant temperature of 0°C and 1330°C is prescribed, respectively.

that various ridge-transform linkage patterns can spontaneously arise from small, initially offset perturbations (weak seeds) in the plate structure. More recent large-strain numerical thermomechanical experiments (Gerya, 2010a, 2013) analyzed spontaneous nucleation and long-term evolution of ridge-transform oceanic spreading patterns. Both analogue and numerical models addressed important aspects of oceanic spreading physics and dynamics (Gerya, 2012 and references therein) but detailed comparison of their predictions with natural data from the incipient oceanic spreading regions remains challenging and requires further effort.

In the present paper, we build on the results of previous numerical experiments (Gerya, 2013) to investigate high-resolution 3D thermomechanical numerical model of incipient oceanic spreading and compare it to observations from the Woodlark Basin. The principal goal of this study is to try to understand detailed dynamics of incipient spreading process and resolve existing controversies for initiation and evolution of ridge-transform oceanic spreading patterns (Lister et al., 1986; Bosworth, 1986; Taylor et al., 1995, 2009).

NUMERICAL MODEL

We use high-resolution 3D petrological-thermo-mechanical numerical model of an incipient oceanic spreading developed in our previous work (Gerya, 2013). The Eulerian-Lagrangian visco-plastic model with an internal free surface (Fig. 1) allows for large degree of plate separation and spontaneous oceanic crust growth by magmatic accretion. Parametric study for this model is presented by Gerya (2013) who investigated its sensitivity to various physical parameters.

The initial model setup corresponds to the onset of oceanic spreading within an already extended (rifted) thin and hot continental lithosphere. In this idealized lithosphere, continental crustal thickness is reduced to 20 km and a geothermal gradient is strongly elevated. In order to initiate offset spreading centers, two linear thermal perturbations (see perturbations A and B in Fig. 1) with an offset of 50 km are imposed at the bottom of the lithospheric mantle (Fig. 1). In order to make model development slightly asymmetric as in the Woodlark Basin, where spreading initiates early in the East than in the West (Taylor et al., 2009), thermal magnitude of the frontal perturbation (see perturbation A in Fig. 2) is taken to be smaller than the magni-

Fig. 2. Conceptual model of oceanic spreading (Gerya, 2013) used in this study. See text for more details.

Sea water Hydrothermal circulation

with percolating melts

tude of the rear one (see perturbation B in Fig. 2). The magnitude of the imposed offset between the perturbations compares well with observations in the Wood-lark Basin (as well as in the other incipient spreading regions) where these offsets are typically in the range of several tens of kilometers (e.g., Bosworth, 1986; Taylor et al., 1995, 2009). The modeled (full) spreading rate (vspreading) is 3.8 cm/yr, which simulates incipient slow- to intermediate-spreading ridge such as in the Woodlark Basin (Taylor et al., 2009). The Eulerian model domain is equivalent to 202 x 98 x 50 km (Fig. 1) and is resolved with a regular rectangular grid of 405 x 197 x 101 nodes and contains 68 million randomly distributed Lagrangian markers. The Eulerian-Lagrangian numerical modeling scheme with open boundaries (Gerya, 2010a, 2013) allows for an infinitely long plate separation. Lagrangian markers leave the Eulerian model domain through the left and right lateral boundaries and enter through the top and the bottom of the model as sea water and asthenospheric mantle markers, respectively. The initial thermal structure of the model with 1000°C isotherm located at the depth of 21 km is shown in Fig. 1. Asthenospheric mantle temperat

Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.

Показать целиком